1. Field of the Invention
This invention relates to communication devices for communication between a host processor and a pocket sized electronic device. More particularly, this invention relates to use of a standard port in the host processor for providing power to a level converter for voltage level conversion necessary for communicating with a palm-top computer and for providing a wake-up signal to turn on the palm-top computer.
2. Discussion of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn. 1.97-1.99
Pocket sized electronic devices such as palm-top computers have proliferated in the last few years. Most pocket devices are CMOS devices using signals levels of 5 volts and 0 volts. Such pocket devices typically use battery based power supplies. One such pocket device is the WIZARD palm-top computer. Technical information about the WIZARD is available from Sharp Electronics Corporation, Sharp Plaza, Mahway, N.J. 07430-2135.
Pocket devices may transmit and receive data serially via a single data communication port. Data communication ports of pocket devices are usually different from the industry standard RS-232 because of the power requirements to meet the RS-232 specification. This is because pocket devices being CMOS based (such as WIZARD) support only signals of 5 V and 0 V. On the other hand, the RS-232 specification requires signals of .+-.12 V. In order for such pocket devices to communicate with host processors, a level converter is needed. Level converters for pocket devices convert signals from the 0-5 V to .+-.12 V and vice versa.
In the past, level converters have been powered from external +12 V and -12 V power sources. However, external power sources may not be necessary if the requisite power can be drawn from a standard port of the host processor. Small peripheral devices such as the mouse draw power from the transmit data (TxD) line and data terminal ready (DTR) line of an industry standard RS-232 port. Although a host port can power small devices, host ports as power sources have the drawback in that power supply fluctuates depending on data transfer activity (TxD line of RS-232 port fluctuates between .+-.12 V when data is transmitted on the line).
Since pocket devices are usually battery based, turning off the pocket device when not in use avoids draining the battery. Thus pocket devices are frequently turned on only for the limited time during which data transfer takes place. In the past, once the pocket device was turned off, user intervention was needed to turn it back on again.
Automatic on-off switching by the level converter avoids cumbersome manual switches and the possibility of a user forgetting to switch off the pocket device. Although in the past it was possible for a pocket device connected to an always on modem to be automatically switched on and off by the modem, the automatic on-off feature was not supported by level converters. This is because in modems the wake-up pin of the pocket device can be driven by a carrier detect signal not available in level converters. Therefore, for level converters, user intervention was necessary to turn on the pocket device after the data transmission was completed and the pocket device turned off in the previous session.
Even for modems, the wake-up feature was not easy to use. In order to take advantage of the wake-up feature, the modem must be powered on and waiting for the phone call from the remote host processor. Furthermore, in order to power off, the remote host processor had to send a shut-down message and disconnect the phone at precisely the same time. If the phone was disconnected too early, the pocket device did not receive the shut-down message. If the phone was disconnected too late, the pocket device powered up again due to the presence of the carrier detect signal on the wake-up pin.